A progressive-power lens includes two viewing zones, each having different power, and one viewing zone therebetween where the power progressively changes. These viewing zones have no boundaries, thereby providing excellent exterior appearance and multiple viewing zones in a single lens, each having different power. For these reasons, a progressive-power lens is frequently used as a spectacle lens with accommodation correction capability for such as presbyopia.
FIG. 3 shows a general construction of a progressive-power lens. FIG. 3(a) is a front view and FIG. 3(b) is a longitudinal cross-sectional view. The progressive-power lens 100 includes an upper distance portion 2 that is a viewing zone for viewing relatively distant objects and a near portion 3 that is located below the distance portion 2 and has different power from that of the distance portion 2 to view relatively near objects. These distance portion 2 and near portion 3 are smoothly connected via an intermediate portion (progressive portion) 4 that is a viewing zone having continuously varying power to view intermediate objects between distant and near objects.
A single-piece spectacle lens needs to make use of an eye-side refractive surface 11 and an object-side refractive surface 12 to acquire all the capabilities required for a spectacle lens, such as vertex power that matches with wearer's dioptric power, cylindrical power for correcting astigmatism, addition power for correcting presbyopia, and prismatic power for correcting heterophoria. Thus, the object-side refractive surface 12 of the conventional progressive-power lens is formed of a progressive-power surface that provides continuously varying power in order to configure the distance portion 2, near portion 3 and intermediate portion 4, while the eye-side refractive surface 11 is used, for example, as the refractive surface for correcting astigmatism.
Such a front surface progressive-power lens having a progressive-power surface on the objective-side refractive surface 12 suffers increased distortion, which may be uncomfortable for first-time progressive-power lens wearers or those who replace their differently designed progressive-power lenses.
To avoid such distortion generation in a front surface progressive-power lens due to the change in image magnification, there has recently been commercialized a back surface progressive-power lens in which a progressive-power surface is formed on the eye-side refractive surface 11, as described in WO 97/19382. As shown in FIG. 3(b), the object-side refractive surface 12 of a back surface progressive-power lens 100 is either spherical or rotationally symmetric aspherical. The eye-side refractive surface 11 is formed of a progressive-power surface having the distance portion 2, near portion 3 and intermediate portion 4. The progressive-power surface is combined with a toroidal surface, or even a complicated surface combined with a correcting aspheric element for correcting off-axis aberrations of the lens. Furthermore, JP-A-2000-227579 describes a technology to reduce the thickness of the back surface progressive-power lens 100.
However, to achieve a curved surface to provide addition power on the back surface, the distance portion of the back surface progressive-power lens has to have surface power greater than that of the near portion by the amount of the addition power. Moreover, the back surface progressive-power lens needs to provide power of distance portion required for the distance portion. For example, when the distance portion is prescribed to have positive power, the surface power of the object-side refractive surface needs to be greater according to the prescribed positive power. Consequently, in the back surface progressive-power lens with the distance portion having the prescribed positive power, the convex object-side refractive surface protrudes more than a corresponding front surface progressive-power lens. Thus, although a back surface progressive-power lens is advantageous in terms of optical performance such as image distortion, it is disadvantageous in terms of the lens thickness and external appearance. The technology capable of reducing the thickness of the lens described in JP-A-2000-227579 is still not enough.
The invention has been made in view of the above circumstances and aims to provide a back surface progressive-power lens capable of solving the disadvantages in terms of lens thickness, external appearance and the like typical of a back surface progressive-power lens in which the eye-side refractive surface is formed of a progressive-power surface.